This dissertation focuses on the behavior of homogeneous FCC metallic interfaces on the nanoscale. Specifically, atomistic calculations (molecular statics and molecular dynamics) with embedded-atom method potentials are used to study the fundamental failure processes that occur at a bicrystal interface in Cu and Al as a result of a mechanical deformation. There are four primary objectives to this dissertation. First, molecular statics calculations are used to determine the most appropriate (minimum energy) structure of homogeneous bicrystal interfaces in Cu and Al. Interface structures and energies are reported in this work, with comparison to both theoretical and experimental characterizations of interface configuration. Second, molecular dynamics simulations are performed to provide a characterization of atomic scale inelastic behavior, including both dislocation and void nucleation activities which lead to interfacial failure. Specifically, two types of interfaces are highlighted in this work: a mirror symmetric interface in aluminum and an asymmetrically dissociated interface in copper. Distorted interface structures (after the dislocation nucleation event) are discussed in terms of partial dislocations or disclinations. Third, molecular dynamics simulations are used to investigate potential relationships between interface structure and interface properties or morphology. The orientation of the primary slip planes with respect to the loading direction and the porosity within the interface region are found to be critical factors in defining the strength of the bicrystal interface, for example. Finally, results of the atomistic calculations are utilized to motivate improved forms for continuum interface separation potentials, ultimately increasing the applicability of these relationships to include cohesive failure in ductile crystalline materials.

<p>Electronic power devices made of silicon carbide promisesuperior performance over today's silicon devices due toinherent material properties. As a result of the material'swide band gap of 3.2eV, high thermal conductivity, itsmechanical and chemical stability and a high critical electricfield, 4H-silicon carbide devices have the potential to be usedat elevated temperatures and in harsh environments. Shortercarrier lifetimes and a reduction in the necessary width of thelow-doped drift zone in silicon carbide devices compared totheir silicon counterparts result in faster switching speedsand lower switching losses and thus in much more efficientpower devices.</p><p>High-voltage 4H-silicon carbide diodes have been fabricatedin a newly developed processing sequence, using standardsilicon process equipment. Epitaxial layers grown by chemicalvapor deposition (CVD) on commercial 4H-silicon carbidesubstrates were used as starting material for both mesa-etchedepitaxial and implanted p+n-n+ planar diodes, Schottky diodesand merged pn-Schottky (MPS) diodes, together with additionaltest structures. The device metallization was optimized to givea low contact resistivity on implanted and epitaxial layers anda sufficiently high Schottky barrier with a singlemetallization scheme. Different high-field termination designshave been tested and breakdown voltages of up to 4 kV onimplanted, field-ring terminated diodes were achieved,corresponding to 80% of the critical electric field. A 5kVepitaxial diode design with a forward voltage drop of 3.5V at acurrent density of 100Acm-2 equipped with an implanted junctiontermination extension (JTE) was also fabricated.</p><p>A new measurement setup was designed and built with thecapability of measuring current-voltage and capacitance-voltagecharacteristics of semiconductor devices at reverse biases upto 10kV. Together with these electrical measurements, theresults of other characterization techniques were used toidentify performance limiting defects in the fabricated siliconcarbide diodes. Increased forward voltage drop of bipolardevices during on-state operation was studied and it was shownthat the stacking faults causing forward degradation arevisible in scanning electron microscopy. With the help ofsynchrotron white-beam X-ray diffraction topographs (SWBXT),electron beam induced current (EBIC) and electroluminescencemeasurements of silicon carbide diodes, the role of screwdislocations as a dominant source of device failure in the formof localized microplasma breakdown was identified. Screwdislocations with and without open core have been found tocause a 20-80% reduction in the critical electric field of4H-silicon carbide diodes, both for low-voltage (150V) andhigh-voltage (~5kV) designs. While micropipes have almost beeneliminated from commercial silicon carbide material,closed-core screw dislocations are still abundant withdensities in the order of 10000cm-2 in state-of-the-art siliconcarbide epitaxial layers.</p>

published_or_final_version / abstract / toc / Mechanical Engineering / Master / Master of Philosophy

No description available.

Large strain tensile tests were carried out on OFHC Cu and 99.99% Al with the aim of determining the first and second order work hardening and rate sensitivity coefficients. The tests were performed at room temperature and 473 K and at constant true strain rates in the range 5 x 10('-4) to 10('-1) s('-1). With the aid of a diameter transducer, which was set up to measure and control the rate of reduction of the diameter of the tensile specimen, the strain rate at the minimum cross-section was held constant well beyond the point of maximum load. A second diametral sensor was constructed for use at elevated temperatures. In order to extend the range of conditions covered, constant strain rate compression tests were also performed on Cu at 698 K. In a further series of experiments, tensile tests were carried out on Cu and Al samples at 293 and on Al specimens at 473 K in which the flow localization process was followed by photographic means. / It was observed that the values of the rate sensitivity of the work hardening rate B(,(sigma)) beyond the maximum load are not negligible, but that they are less than 1, in opposition to the theoretical predictions of Kocks et al('(47)). Furthermore, it is shown that, contrary to the suggestion of these workers, the rate sensitivity at constant work hardening rate N is not the material coefficient that controls the growth of strain rate gradients at large strains. / The material coefficients determined using the diametral transducer were employed for the numerical integration of the second order differential equation describing flow localization proposed by Kocks et al('(47)). This equation was integrated at the minimum cross-section of the sample, and the solution is compared with the one calculated by integrating the first order differential equation proposed earlier by Jonas et al('(10)). As expected, the strain measurements obtained from the flow localization experiments are reproduced more closely by the second order solution than by the first order one largely because of the non-negligible values of B(,(sigma)). However, at large deformations, there is a discrepancy between the experimental observations and the predictions of the second order theory. This is attributed to the development of triaxial stresses at these strains. A possible modification of the second order treatment is suggested, based on the gradient in the Bridgman correction term.

No description available.

Recent short pulse (femtosecond) laser experiments have shown the existence of a so called superelastic precursor for short time periods after shock wave formation. The superelastic precursor is characterised as having amplitude far greater than the Hugoniot Elastic limit. This work reviews the current orthotropic thermoelastic plastic-damage model developed at Cranfield University, which includes the ability to model high velocity, shock wave forming impacts. The current model is unable to reproduce the superelastic precursor. Recent methods of looking at plasticity are reviewed and model improvements are suggested to enable the Cranfield model to reproduce superelastic precursor waves. The methods investigated are both dislocation based as it is determined that it is necessary to model deformation on the microscale in order to achieve reproduction of phenomena on the timescales of the early stages of shock wave formation and propagation. The methods investigated are the so-called self-organisation of dislocations and a mobile and immobile dislocation method proposed by Mayer. The plasticity part of the model proposed by Mayer is suggested for further investigation, including implementation into the DYNA 3D hydrocode which contains the current Cranfield model, to numerically asses the models capabilities. Similar, the self-organisation model is put forward for further numerical analysis. Further, calculation of the continuum Cauchy stress using purely atomistic variables is investigated in the form of the virial stress. It is determined that the virial stress calculation is unsuitable for modelling shock waves, however an alternative atomistic stress calculation which is more suited to shock waves is discussed. It is proposed that this stress calculation could be used to investigate the stresses contained within the thin shock front.

HREM imaging was performed using the Jeol 4000ex microscope on specimens prepared from an as-grown ingot of semi-insulating Gallium Arsenide. Various low angle grain boundaries were imaged in the [110] orientation, misorientations varying between 4°-13°. Detailed study of a grain boundary of 11.5° misorientation about the [110] rotation axis has been carried out. Burgers vector analysis showed the presence of perfect 60° and [001] dislocations. Modelling of the [001] dislocation has been carried out using the Tersoff potential, Bond Order Potential and a tight binding Hamiltonian for GaAs, using Chadi (1984) parameters. The dislocation core was associated with an 8-membered and two 5-membered rings. Assum- ing there is a minimum of wrong bonds, we predict that the core has two wrong bonds, one being Ga-Ga, and the other As-As, both in equivalent positions where the two 5-membered rings were appended to the 8-membered ring. The Ga-Ga bond is considerably shorter and hence stronger than the As-As bond. Band structure calculations performed using a Vogl (1983) sp³s* Hamiltonian revealed deep states in the gap, which are associated with atoms in the core only. Using Stadelmann's (1987) EMS program, successful image matching of calculated images of the [001] dislocation has been achieved with the experimental image, using the atomic structure generated by tight binding relaxation. Ga and As being only two atomic numbers apart have similar scattering factors and cannot be easily distinguished in the experimental image. The equivalence of the position of the two wrong bonds greatly eases image matching as it is no longer necessary to know which is the Ga-Ga , and which is the As-As bond. This is the first suggested model of the [001] dislocation in GaAs, to the best of my knowledge. It is found to be similar to the atomic structure of the 90° partial dislocation in silicon (Bigger et al., 1992). No account of segregation of impurities to the grain boundary, or the [001] dislocation core is taken here, though it is very likely that an impurity atom would sit itself in this large space. The relaxed atomic structure for the 60° dislocation showed a doubling of periodicity along the dislocation line, similar to that found in the 30° partial in Si. The core consists of a 7-membered and a 5-membered ring with a minimum of two wrong bonds. In addition to this, quantitative comparisons of the [001] HREM image and simulated structures have been made and an iterative structure refinement carried out in order to achieve the best image matching. The resultant 'experimental-best-fit' structure was not found to be physically or chemically plausible.

Thesis (M.App.Sc.) - Carleton University, 2005. / Includes bibliographical references (p. 121-125). Also available in electronic format on the Internet.

Thesis (Ph. D.)--University of Washington, 1990. / Vita. Includes bibliographical references (leaves [146]-151).